What Are Carcinogens

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Cancer causing

Medical Specialties:

Family practice, Internal medicine, Oncology, Radiology

Clinical Definition:

A carcinogen is an agent or substance that may damage cellular DNA or alter cell metabolism, and, in so doing, potentially lead to the development of cancer. Carcinogens are classified according to their carcinogenicity.

In Our Own Words:

A carcinogen is something believed to contribute to the development of cancer.

Usually, a carcinogen either causes a cell to divide too quickly or too haphazardly, or in some other way it leads to changes in the DNA imprint of a cell.

Not all carcinogens have the same potential to cause cancer. Various government agencies, such as the Environmental Protection Agency, classify carcinogens depending on how likely they are to lead to cancer. For instance, certain carcinogens (for example, cigarette smoke or formaldehyde) require long-term exposure before they cause cancer.

More Information About Carcinogens

The term carcinogen is broad and refers to a variety of viral, chemical or physical agents.

Carcinogens are one of three types depending on how they elicit tumor formation:

  • Genotoxins are chemical agents and cause tumor formation by causing mutation.
  • Tumor promoters enhance tumor formation after exposure to genotoxins.
  • Similarly, cocarcinogens potentiate cancer formation by boosting the effect of genotoxins.

    In 1761, John Hill noticed an association between the use of tobacco snuff and nasal cancer; thus, tobacco was the first identified carcinogen.

    The International Agency for Research on Cancer (IARC) currently categorizes and rates more than 100 chemical agents as identified carcinogens.

    More specifically, the IARC classifies agents into 5 groups:

    • Group 1 agents are known as carcinogens based on epidemiological studies examining people who have been exposed to these agents for years.
    • Group 2A agents are probable carcinogens that have proven to be cancerous to animals under conditions that are also experienced by humans.
    • Group 2B agents are possible carcinogens that have been observed to be associated with malignant tumors in one or more animal species.
    • Group 3 agents are not classifiable.
    • Group 4 agents are probably not carcinogens.

    When most people hear the term "carcinogens," the capacity of these chemical agents to affect the fetus and cause birth abnormalities first comes to mind. However, fewer than 20 percent of birth defects are linked to carcinogens. Furthermore, the human fetus is most susceptible to the effect of carcinogens between the third and eighth week of pregnancy and pregnancy lasts 40 weeks.

    About 60 to 90 percent of cancers are thought to be attributable to environmental factors or carcinogens.

    Here is a list of environmental factors that are linked to cancer:

    • workplace exposures
    • naturally occurring exposures (for example, infection, sunlight, radon and so forth)
    • medical treatments (for example, radiation therapy or chemotherapy)
    • pollution
    • household exposures

    In the workplace, some examples of carcinogens include mercury, carbon monoxide, lead and cigarette smoke.


    American Cancer Society. “Known and Probable Human Carcinogens.” Accessed July 2013. 

    National Cancer Institute. “Dictionary of Cancer Terms.” Accessed July 2013.

    Environmental Protection Agency. “Risk Assessment for Carcinogens.” Accessed July 2013.

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